What is the equivalent of an Objective-C id in Swift?

Question

Swift 3

Any, if you know the sender is never nil.

@IBAction func buttonClicked(sender : Any) {
    println("Button was clicked", sender)
}

Any?, if the sender could be nil.

@IBAction func buttonClicked(sender : Any?) {
    println("Button was clicked", sender)
}

AnyObject, if you know the sender is never nil.

@IBAction func buttonClicked(sender : AnyObject) {
    println("Button was clicked", sender)
}

AnyObject?, if the sender could be nil.

@IBAction func buttonClicked(sender : AnyObject?) {
    println("Button was clicked", sender)
}